DWELLERS BETWEEN TIDE MARKS
THE COLONIAL MOSS ANIMALS
Seaweeds and rocks at and below the limit of the ebbing tide are often
covered with small bushy growths, or with lacelike incrustations that
are alive. These are moss animals, representing the class Polyzoa of
the phylum Molluscoida. They are minute, soft creatures that live in
colonies formed by the repeated budding of the members, all connected
by a fleshy base so that each contributes to the nourishment of all.
"Each little animal occupies a separate stony or horny capsule, into
which it may withdraw and even close the opening with a lid.... The
mouth is surrounded by tentacles that in many species arise from a
horseshoe-shaped or disklike base. These tentacles are always beset
with hairlike bristles which by their movements serve to set up
currents, and thus to drive minute organisms into the mouth."
A typical example of these polyzoans (or bryozoans) is _Bugula
turrita_, so abundant wherever our northeastern coast is rocky that
the rocks below tide level appear covered with its mossy tufts, which
are often ten inches long and profusely branched. The main stems are
orange-yellow, while the terminal branches are yellowish white. The
delicate tracery so frequently seen on the fronds of kelp, and on
shells and stones along both shores of the Atlantic indicate colonies,
or their remains, of the lace coralline (Membranipora); and the dull
red or pinkish crust so common on shells and stones in shaded tidepools
represents successive colonies of the "red-crust" polyzoan (_Escharella
variabilis_), layer crusting over layer. A similar history accounts
for the curious nodules called "false coral" so common in moderately
deep water in Long Island Sound. Similar polyzoans, which exist in
great variety, both modern and fossil, contributed extensively to the
formation of the older strata of sedimentary limestones.
ANCIENT LAMP SHELLS
Associated in structure with these minute colonists is the ancient race
of brachiopods (Brachiopoda, "arm-footed") or lamp shells, although
they much more nearly resemble bivalved mollusks, whence, by the way,
comes the name of the phylum to which both belong--Molluscoida, which
means "mollusklike."
The race of the brachiopods goes back to the beginning of the geologic
record. A few living examples are still found in the ocean, some of
which, as lingula, have changed so little that they can hardly be told
from the most ancient fossils of their family. Certain species are
dredged abundantly on both coasts of the Atlantic from water a few
fathoms deep where the bottom is rocky. They look like small mussels at
first sight, but on examination show a vast difference in structure.
The bivalve shells, instead of growing on the right and left sides of
the animal, as in bivalve mollusks, cover its back and front, and the
head parts are at the gape of the valves. At the hinge end of the shell
the lower valve overlaps (it is the shape of this lower shell, like
that of an old Roman lamp, which suggests their common name, "lamp
shells") and the hinder end of the body projects as a stalk, by which
the animal fastens itself to the rock. "The mouth in the brachiopods is
flanked by two curiously coiled and feathered arms which lie within the
cavity between the shells, and are supported by skeletal rods attached
to the upper shell. These serve as gills, and also to capture the
minute creatures upon which the brachiopod feeds."
Owing to their great abundance, world-wide distribution, and remote
antiquity, as well as their excellent state of preservation,
brachiopods occupy a very conspicuous rank among extinct invertebrates,
and furnish us besides with a large number of important index fossils.
They are to be found in immense variety from the Cambrian to the
present, most numerously in formations from Silurian to Permian times.
[Illustration: STARFISH AND OTHER TYPICAL LIFE IN A TIDE POOL
Photo, American Museum of Natural History]
STARFISHES, SEA URCHINS, AND TREPANGS
We have now arrived at the point (phylum Echinodermata,
"spiny-skinned") where a distinctly new type of interior structure
appears in the possession by animals of a hollow space (coelom) between
the outer skin and the wall of the digestive tube which now becomes
occupied by definite organs instead of by an almost uniform mesenchyme,
as in the sponges and coelenterates. These organs arise from an
interior lining membrane called "mesoderm."
Henceforth, therefore, we shall deal with coelomate animals, among
which the echinoderms are lowest in rank. The simplest of them is the
"sea lily" which lives rooted on the bottom in deep water, and sways
about on a slender, jointed stalk, looking much like the flower after
which it is named. It is of interest chiefly as a survivor of the tribe
of crinoids that were so varied and numerous in early Paleozic times
that massive Devonian limestones are composed largely of their remains;
and the type has changed little through the ages. It consists typically
of a cup, mounted on its stem like the calyx of a flower, and composed
of circles of calcareous plates, definite in form and in relative
position, that contain and protect a well-organized body. Surrounding
the open mouth of the cup is a circle of long, jointed, much-branched
tentacles that sweep the water, capture passing prey, and bring it into
the mouth of the crinoid within the circling base of the arms.
[Illustration: CORAL FORMATIONS OF THE GREAT BARRIER REEF, AUSTRALIA
Photo, American Museum of Natural History]
If now you were to cut off its stalk, lay the crinoid on the sand,
mouth down and arms outspread, beside a brittle star or a basket fish,
which also have many-branched arms, it might be difficult to tell them
apart, yet they represent different orders; and from this, by way of
the naked serpent star, it is but a short transition to the starfish,
where the arms are no longer tentaclelike, but are simply pointed
extensions of a central body; this, in fact, is the case, for now they
are no longer prehensile organs, but are supports, mainly serviceable
in locomotion, and the stomach and ovaries are partly lodged in them.
The main point just now, however, is the fact that here, and in the
successive changes of form to be shown, the pattern of plates that form
a strengthening mosaic in the skin of the central part of the body
remains identical.
All starfishes are not as prettily symmetric as our familiar
five-finger. Some are shorter in the arms, and much broader and
thicker in the body; and if you will examine a collection of preserved
specimens of the echinoderms you will see that you can trace gradation
of form right around to the bun-shaped cake urchin, on whose top the
five-pointed star is printed, and thence to the globular sea egg,
which the French called "sea urchin," using one of their names for the
hedgehog. Furthermore, the five sections of the shell of the urchin,
which represent the five arms of the starfish folded forward and grown
together into a spherical case, are to be traced again, outlined by
appendages, in the elongated and leathery hide of the trepangs and sea
cucumbers of the order Holothuria.
It is as an illustration of homology, that is, the resemblance between
parts that have the same relation to the typical plan of structure,
and as an example of how almost endless variations of form may arise
within a single type, that the echinoderms are of most interest.
Otherwise it may be said that they serve as food for fishes and some
other creatures, including coastwise savages, and as curiosities in
geological museums and in aquaria; and that starfishes are sadly
destructive of cultivated oyster beds. We may therefore dismiss them,
and devote a page or two to the worms.
EARTHWORMS AND BEACHWORMS
Although various parasitic creatures have been described as flat
"worms," round "worms," and so forth, naturalists regard as true worms
only those of higher organization classified in the phylum Annulata, or
annelids, the distinctive characteristic of which is that its members
have elongated bodies divided into ringlike sections. These represent
a division of the internal parts into a series of structural segments
or "matemeres," each supplied with its own set of organs, yet connected
by blood vessels and nerves, and the whole traversed by tubular organs
serviceable to the entire animal. The nervous system consists of a
"brain" in the head, and a double, ventral nerve-cord with a ganglion
in every segment, foreshadowing the nervous system in insects and other
arthropods. The phylum embraces three classes: 1. Chætopoda--earthworms
and marine annelids; 2. Gephyrea--marine worms, otherwise called
sipunculoids; and 3. Hirudinidæ--leeches.
The earthworm or "angleworm" (that is, angler's worm, bait worm)
of the "common garden variety," to use the phrase of old-fashioned
encyclopedias, is a typical example of the first class, whose Latin
name refers to the bristles (setæ) on the flattened lower surface of
the body that serve the worm as "feet." A magnifying glass shows them
in four double rows allowing eight to each of the rings into which the
body is so plainly divided; their extremities are directed rearward,
and by their means the worm pushes itself along, and is able to cling
to and climb not only the walls of its burrow but vertical surfaces
when not too smooth. Thus they are found frequently on roofs and in
other elevated and surprising places, to which they have crawled in
the night, when, as well as in warm, rainy weather, they are likely to
wander a great deal. The long and greatly extensible and elastic body
tapers almost equally at each end, but the head end is that which goes
forward in crawling, and a lens will show a mouth on its lower surface,
beneath a sort of thick lip. A long gullet leads into an expansion
called the crop, and that into a large, tough-walled stomach, beyond
which an intestine leads to the last segment. The thirty-third to
thirty-seventh segments are swollen, forming the "belt" (clitellum),
which denotes maturity, but seems to have no special functions. The
senses are few and dull. No eyes exist, nor sense of hearing, but
the skin is extremely sensitive to vibrations, and to bright light,
as might be expected in a nocturnal animal. The sense of taste is
discriminating. The eggs are extruded in such a way as to form a
glutinous ring about the body, which, when complete, is slipped over
the head of the worm, and left to hatch in warm soil under a stone.
Earthworms live underground in burrows that are sunk well below the
frost line. In digging they work head downward, gnawing--although they
have no hard jaws--and swallowing the earth that is not easily crowded
aside and then throwing it out and perhaps heaping it up as "castings."
The tunnel must be wide enough to let its occupant turn around in it,
and it ends in a deep chamber in which one or more worms may pass the
winter without freezing. These worms naturally seek a loose, damp soil,
not only for ease of working, but because moisture is a necessity,
as they breathe through their skin; hence they abound in meadows and
cultivated soil, and are not found on high, dry plains. During the
day they lie near the surface, often with the head just protruding.
Here they are discovered by sharp-eyed birds and garter snakes, and
sacrificed by thousands, notwithstanding the strength with which they
hang on to their retreats by the tail. When it retires to the depths of
its burrow this worm plugs the mouth of the tunnel with leaves which
it draws always by the base, exhibiting considerable intelligence in
manipulating the various shapes of leaves to that end.
The world-wide distribution of the earthworm is to some extent owing
to man's agency. On our northwestern plains, for example, these worms
originally were absent, but are now widely distributed and flourishing
there, having been carried from the east, as eggs or small worms, in
the soil packed about the roots of trees and shrubs transplanted to
western orchards and gardens. This fact may have something to do with
the recent westward spread of the robin, which, more than any other of
our birds, is a hunter of them. Except where excessively numerous these
worms do far more good than harm in a garden.
The naids (Naidæ) are small transparent worms that creep about on
vegetation in fresh water, and, besides laying large eggs, they
occasionally divide into two at a place in the body that appears
arranged for this purpose, for it consists of a zone of very elementary
tissue. "Gradually," as Minot records, "the tissue of this interpolated
zone transforms itself into muscles, nerves, etc., and, growing
meanwhile, it forms in front a new tailpiece to patch out the anterior
half of the worm, and behind it forms a new head for the posterior
half of the original body. The zone then breaks and there are now two
worms." A relative, the lumbriculus, does the trick in a much more
prosaic way, breaking in two first, and letting the separate halves
acquire head or tail as best they may. This ability to reproduce lost
parts is of much service in the life of the species and often of the
individual, which may still live after some water tiger has bitten it
in two--and these worms are at the base of the food supply of rivers
and ponds, and would soon be exterminated were they not capable of
rapid and profuse multiplication.
Worms of this class dwell in great numbers and variety in the sea and
in salt-water meadows and beaches, and are often beautiful as well as
interesting objects of study for the visitor at the shore. The sea
mouse (Aphrodite), for instance, which is about three inches long and
of oval shape, is covered with hairlike bristles that glisten with
brilliant green, red, and yellow iridescence; it is to be looked for on
the mud just below the low-tide line, and inhabits both coasts of the
North Atlantic. The body of the common "clay worm," dug for bait at low
tide, which is olive in general tone, gleams with pearly iridescence,
while its innumerable feet bear gills that are green and salmon-red.
Another (Lumbriconereis) is known as "opal worm" for good reason; and
our sands abound in slender scarlet worms of the same genus named
"red thread." All these worms bury themselves in the sand, or wander
through it in search of prey, for they are carnivorous, and do not
hesitate to kill and eat each other. Some are fairly sedentary, and
protect themselves against fishes, crabs, mollusks, and bigger annelids
that seek them, by forming tubes by means in some cases of a shelly
secretion, but more usually by cementing bits of shell, stones, and
grains of sand into an irregular tube lining the burrow; the slender,
limy serpentine tubes often seen on stones or dead shells in tide
pools, are, or were, the homes of such protected worms, most commonly
of the "shell worm" (Serpula). "Often a number of these calcareous worm
tubes are seen clustered together. When undisturbed the worm protrudes
its beautiful feathered gills, which resemble a little passion flower
projecting from the mouth of the tube. These gills are variously
colored in different individuals, some being purplish brown, banded
with white and yellow, while others are yellowish green, orange, or
lemon-yellow. At the least disturbance, such as a shock or a shadow,
the gills are instantly withdrawn into the stony tube, and the opening
stopped by a horny disk." In the Gulf of Mexico extensive colonies of
these worms often form, and as the early generations die others erect
their tubes above them; as this goes on sand and shell fragments fill
around and between the tubes, and after a long time the whole mass
becomes a solid reddish, loose-lying rock, composed chiefly of serpula
tubes, which in Florida is dragged up from the beach and used as
building stone.
The third class (Hirudinidæ) of Annulata is that of the leeches, those
ugly, but useful, worms of land and sea. In spite of their sluglike
appearance the leeches are segmented worms, although the wrinkles on
their gray, mottled skins do not indicate the position of the segments
beneath. The mouth on the under side of the head is armed with jaws
and sharp teeth that make three or more cuts through the skin, whence
the blood is sucked; there is also a holding sucker near the tail.
Their attacks cause little pain, and that fact has led physicians to
put them into use when bleeding is required. The eggs of leeches are
laid in moist earth in little packets, and hatch in five or six weeks.
The growth to maturity is slow, and continues during a long life. Many
species abound in ponds and stagnant waters. Asia has terrestrial
leeches, swarming in moist vegetation; and in Ceylon the minute
leeches are a terrible plague in certain regions. Many also are wholly
marine. Some of the larger forms attack fishes directly, and quickly
kill them by sucking their blood away; others are true parasites. On
the other hand the leeches of our lakes are fed on by the whitefish and
similar fishes. They are a great pest to our fresh-water turtles.